Differential Die-away Analysis Research Articles

Development of an integrated non-destructive analysis system, Active-N

ABSTRACT An integrated active neutron non-destructive analysis (NDA) system, Active-N, was developed to gain knowledge of active neutron NDA techniques that are applicable to measurements of nuclear materials in highly radioactive nuclear fuels. Active-N, equipped with a D-T neutron generator, combines three complementary active neutron NDA techniques: Differential Die-away Analysis (DDA), Prompt Gamma-ray Analysis (PGA), and Neutron Resonance Transmission Analysis (NRTA). In this paper, we provide an overview of Active-N and then demonstrate that the compact NRTA system in Active-N can quantify nuclear materials. Monte Carlo simulations were conducted to determine the design of the compact NRTA system including a moderator, flight tubes, and a detector shield. To investigate how accurately the compact NRTA system determines areal number densities in a sample, measurements were performed with a Pu pellet-type sample as well as metallic plate samples of In and Ag. The experimental areal number densities of 240Pu, 115In and 109Ag were consistent with those calculated for the individual nuclei. These results show that it is feasible to develop a compact NRTA system capable of determining the contents of nuclear materials in nuclear fuels.

Differential die-away technology applied to detect special nuclear materials

Differential die-away analysis (DDAA) technology is a special nuclear material (SNM) active detection analysis technology. Be a nuclear material shielded or not, the technology can reveal the existence of nuclear materials by inducing fission from an external pulsed neutron source. In this paper, a detection model based on DDAA analysis technology was established by geant4 Monte Carlo simulation software, and the optimal sensitivity of the detection system is achieved by optimizing different configurations. After the geant4 simulation and optimization, a prototype was established, and experimental research was carried out. The result shows that the prototype can detect 200 g of 235U in a steel cylinder shield that's of 1.5 cm in inner diameter, 10 cm in thickness and 280 kg in weight.

Improved evaluation of safeguards parameters from spent fuel measurements with the Differential Die-Away (DDA) instrument

The Differential Die-Away (DDA) technique is a highly sensitive non-destructive assay method for characterizing and detecting the presence of fissile material within an item of interest. DDA utilizes a series of pulses from a neutron generator (NG) to actively interrogate an item of interest. The die-away time of the neutron population induced by this active interrogation and the integral of the total differential die-away signal can be used to characterize items such as nuclear waste drums and spent nuclear fuel assemblies. Los Alamos National Laboratory (LANL) conceptualized, designed, and fabricated a DDA instrument that was deployed for field test measurements at the Central Interim Storage Facility for Spent Nuclear Fuel (Clab) in Oskarshamn, Sweden. The instrument performed multiple static measurements at fixed locations and dynamic axial scans of 15 pressurized water reactor (PWR) and 10 boiling water reactor (BWR) spent fuel assemblies, collecting both passive and active measurement data. The static assays of the assemblies measured the differential die-away signal, die-away time, and total passive neutron emission rate to create calibration curves for the evaluation of assembly multiplication, burnup, initial enrichment, effective fissile mass, and total elemental plutonium mass. Each calibration curve was optimized by minimizing the relative root mean square error (RRMSE) of assembly assay results compared to declared assembly parameters. The same quantities were also measured with the axial scans, and the resulting data were applied in two ways: (1) in the creation of calibration curves to improve evaluation of the same safeguards parameters as static assays, and (2) for comparison to simulation. In most cases, across both PWR and BWR assemblies, axial scan data improved the estimation of the above parameters, quantified by decreasing the calibration curve RRMSE. These axial scan results demonstrate the ability of the DDA instrument and analysis method to characterize spent PWR and BWR fuel as well as, or better than, a static assay of the same assembly. Furthermore, the DDA instrument’s unique ability to obtain both active and passive data in a single, axial scan of an entire spent fuel assembly represents a more efficient and accurate way of assaying spent fuel for verification purposes. These results represent a significant advancement for characterizing spent nuclear fuel compared to current technologies.

Sub-critical assembly die-away analysis with organic scintillators

Determining the reactivity of systems containing fissile material is essential for verifying safety, and possession and enrichment levels. The goal of this experiment was to characterize a subcritical nuclear fuel assembly irradiated by a pulsed neutron generator through differential die-away analysis. Similar characterizations are commonly performed with thermal neutron detectors, but these detectors are bulky and cumbersome, the materials are not widely available, and timing can be clouded by the required thermalization time. In this work, an organic scintillator capable of detecting both neutrons and photons was used to measure the die-away profiles of both particles. These time profiles were then used to estimate the reactivity of the assembly. The calculated reactivity estimates compare favorably to simulated estimates, and therefore provide a useful tool for fissile material characterization.

Neutron Slowing Down Time Based Inspection Method

The neutron slowing down time based inspection system extends the well-established differential die-away analysis (DDAA) to higher neutron energies and earlier fission neutron detection times. Simulation tools, mainly MCNPX, validated previously by experiments are used to further extend the domain of DDAA, which relies solely on thermal neutron-induced fissions, into the high-sub-MeV neutrons. Detailed time–energy correlated behavior of slowing-down neutrons, following a narrow pulse of 14-MeV neutrons, is calculated for ideal moderators like polyethylene and beryllium as well as for generic hydrogenous and metallic cargo materials. The time evolution and energy spectra of neutrons escaping the cargo-containing special nuclear materials (SNMs) (such as 233U, 235U, 239Pu, and 241Pu) are shown to be very different and easily separable from the same quantities when an SNM is absent. The key to the technique is the “vanishing time”: the time when high-energy source neutrons are scattered down to energies below those of fission neutrons. If fissile material is present, high-energy neutrons (>1 MeV) reappear solely due to fissions induced by the slowed down source neutrons, with energy below 1 MeV. The fission neutrons, which escape cargo absorption, are then detected by a direct fast neutron detector. The detectors can be, in principle, organic or 4He recoil-based scintillators, which are inherently insensitive to neutrons with lower energy than the fission neutrons. In addition, a successful implementation of this SNM inspection technique requires an intense medium-to-high duty factor narrow-pulse fast neutron generator, e.g., (d, T) or (d, D), and fast two-parameter (energy and time) data digitizers.

Differential Die-Away Analysis for detection of 235U in metallic matrix

Differential Die-Away Analysis is a powerful tool for detecting small quantity of fissile material even if it is shielded or placed in non-fissile matrix. The technique can be used to monitor and characterize fissile content for nuclear waste assay. In this paper, we have discussed the application of differential die away technique for detection of small quantity of fissile material in nuclear waste assay. Feasibility experiments to optimize various parameters have been carried out for detection of 235U in metallic matrix and reported in this paper. A minimum quantity of 1g of 235U in 150kg of metallic matrix has been detected in the experimental configuration being reported.

Neutron kinetics in moderators and SNM detection through epithermal-neutron-induced fissions

Extension of the well-established Differential Die Away Analysis (DDAA) into a faster time domain, where more penetrating epithermal neutrons induce fissions, is proposed and demonstrated via simulations and experiments. In the proposed method the fissions stimulated by thermal, epithermal and even higher-energy neutrons are measured after injection of a narrow pulse of high-energy 14MeV (d,T) or 2.5MeV (d,D) source neutrons, appropriately moderated. The ability to measure these fissions stems from the inherent correlation of neutron energy and time (“E–T” correlation) during the process of slowing down of high-energy source neutrons in common moderating materials such as hydrogenous compounds (e.g., polyethylene), heavy water, beryllium and graphite. The kinetic behavior following injection of a delta-function-shaped pulse (in time) of 14MeV neutrons into such moderators is studied employing MCNPX simulations and, when applicable, some simple “one-group” models. These calculations served as a guide for the design of a source moderator which was used in experiments. Qualitative relationships between slowing-down time after the pulse and the prevailing neutron energy are discussed. A laboratory system consisting of a 14MeV neutron generator, a polyethylene-reflected Be moderator, a liquid scintillator with pulse-shape discrimination (PSD) and a two-parameter E–T data acquisition system was set up to measure prompt neutron and delayed gamma-ray fission signatures in a 19.5% enriched LEU sample. The measured time behavior of thermal and epithermal neutron fission signals agreed well with the detailed simulations. The laboratory system can readily be redesigned and deployed as a mobile inspection system for SNM in, e.g., cars and vans. A strong pulsed neutron generator with narrow pulse (<75ns) at a reasonably high pulse frequency could make the high-energy neutron induced fission modality a realizable SNM detection technique.

Fissile mass estimation by pulsed neutron source interrogation

Passive methods for detecting correlated neutrons from spontaneous fissions (e.g. multiplicity and SVM) are widely used for fissile mass estimations. These methods can be used for fissile materials that emit a significant amount of fission neutrons (like plutonium). Active interrogation, in which fissions are induced in the tested material by an external continuous source or by a pulsed neutron source, has the potential advantages of fast measurement, alongside independence of the spontaneous fissions of the tested fissile material, thus enabling uranium measurement.Until recently, using the multiplicity method, for uranium mass estimation, was possible only for active interrogation made with continues neutron source. Pulsed active neutron interrogation measurements were analyzed with techniques, e.g. differential die away analysis (DDA), which ignore or implicitly include the multiplicity effect (self-induced fission chains).Recently, both, the multiplicity and the SVM techniques, were theoretically extended for analyzing active fissile mass measurements, made by a pulsed neutron source. In this study the SVM technique for pulsed neutron source is experimentally examined, for the first time. The measurements were conducted at the PUNITA facility of the Joint Research Centre in Ispra, Italy. First promising results, of mass estimation by the SVM technique using a pulsed neutron source, are presented.

MCNP Simulation Benchmarks for a Portable Inspection System for Narcotics, Explosives, and Nuclear Material Detection

MCNPX simulations have been used to guide the development of a portable inspection system for narcotics, explosives, and special nuclear material (SNM) detection. The system seeks to address these threats to national security by utilizing a high-yield, compact neutron source to actively interrogate the threats and produce characteristic signatures that can then be detected by radiation detectors. The portability of the system enables rapid deployment and proximity to threats concealed in small spaces. Both dD and dT electronic neutron generators (ENG) were used to interrogate ammonium nitrate fuel oil (ANFO) and cocaine hydrochloride, and the detector response of NaI, CsI, and LaBr3 were compared. The effect of tungsten shielding on the neutron flux in the gamma ray detectors was investigated, while carbon, beryllium, and polyethylene ENG moderator materials were optimized by determining the reaction rate density in the threats. In order to benchmark the modeling results, experimental measurements are compared with MCNPX simulations. In addition, the efficiency and die-away time of a portable differential die-away analysis (DDAA) detector using 3He proportional counters for SNM detection has been determined.

Special Nuclear Materials Detection Using IEC Fusion Pulsed Neutron Source

Special Nuclear Materials (SNM) detection efforts have largely been divided into two main groups: active and passive. Passive techniques are highly desirable in that a radiation source need not be employed in order to detect fissile materials which broadcast a clear radiative signature. However, disadvantages can be seen in HEU (Highly Enriched Uranium) detection, for example, where the system’s efficacy is limited by its ability to detect a weak self-radiative signature from U. Active interrogation provides a catalyst for amplifying HEU’s presence vis-a-vis fission event inducement, which in turn yields a starker signature which can be discerned through an understanding of fissile materials and neutron transport in various media. Ongoing work in the Fusion Technology Institute’s Inertial Electrostatic Confinement (IEC) Experiment has focused on using the pulsed D-D neutrons from an IEC to interrogate the presence of HEU in an enclosed space. The paper begins with a brief description of the neutron-based detection schemes of Delayed Neutron Analysis (DNA) and Differential Die-Away (DDA). Experimental delayed neutron counts of ninety above the background at an interrogating neutron flux of 5.5x104 n/cm2-s are seen to confirm MCNP modeling results. MCNP is also utilized to probe future concepts in neutron-based active interrogating SNM detection systems using DDA analysis.

Fissile and fertile nuclear material measurements using a new differential die-away self-interrogation technique

This paper presents a new technique for the measurement of fissile and fertile nuclear materials in spent fuel and plutonium-laden materials such as mixed oxide (MOX) fuel. The technique, called differential die-away self-interrogation, is similar to traditional differential die-away analysis, but it does not require a pulsed neutron generator or pulsed beam accelerator, and it can measure the fertile mass in addition to the fissile mass. The new method uses the spontaneous fission neutrons from 244Cm in spent fuel and 240Pu effective neutrons in MOX as the “pulsed” neutron source, with an average of ∼2.7 neutrons per pulse. The time-correlated neutrons from the spontaneous fission and the subsequent induced fissions are analyzed as a function of time to determine the spontaneous fission rate, the induced fast-neutron fissions, and the induced thermal-neutron fissions. The fissile mass is determined from the induced thermal-neutron fissions that are produced by reflected thermal neutrons that originated from the spontaneous fission reaction. The sensitivity of the fissile mass measurement is enhanced by the use of two measurements, with and without a cadmium liner between the sample and a hydrogenous moderator that surrounds the sample. The fertile mass is determined from the multiplicity analysis of the neutrons detected soon after the initial triggering neutron is detected. The method obtains good sensitivity by the optimal design of two different neutron die-away regions: a short die-away for the neutron detector region and a longer die-away for the sample interrogation region.

Differential die-away analysis system response modeling and detector design

Differential die-away-analysis (DDAA) is a sensitive technique to detect presence of fissile materials such as U235 and Pu239. DDAA uses a high-energy (14MeV) pulsed neutron generator to interrogate a shipping container. The signature is a fast neutron signal hundreds of microseconds after the cessation of the neutron pulse. This fast neutron signal has decay time identical to the thermal neutron diffusion decay time of the inspected cargo. The theoretical aspects of a cargo inspection system based on the differential die-away technique are explored. A detailed mathematical model of the system is developed, and experimental results validating this model are presented.

Pulsed neutron differential die away analysis for detection of nuclear materials

Differential die away analysis (DDAA) is a very sensitive technique for detecting the presence of fissile materials such as 235U and 239Pu. DDAA is based on conventional pulsed neutron source interrogation, employing typically low cost 14MeV (d,T) generators. In DDAA, a neutron generator produces repetitive pulses of neutrons that are directed into an inspected cargo. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The population of thermal neutrons decays with the diffusion decay time of the inspected medium (the so called thermal neutron “die-away” time) on the order of hundreds of microseconds. If SNM is present, the thermalized neutrons from the source cause fissions that produce a new source of fast neutrons. These fast fission neutrons decay with a time very similar to that of the thermal neutron die away of the surrounding cargo. The sensitivity of DDAA for a given source of neutrons is greatly affected by the size, geometry, density and composition of the inspected cargo. The sensitivity is also affected by the reflection of neutrons from surfaces such as the ground, nearby walls or nearby cargo; these effects were studied. The ability of DDAA to detect enriched uranium sample in dense hydrogenous cargo, such as paper and wood is shown.

Improving differential die-away analysis via the use of neutron poisons in detectors

Differential Die-Away Analysis (DDAA) is an active interrogation technique to detect special nuclear material (SNM). In DDAA, a pulsed neutron generator produces pulses of neutrons that are directed into a cargo to be interrogated. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. If SNM is present, the thermalized neutrons from the source will cause fissions that produce a new source of neutrons. The number of thermal neutrons decay exponentially with the diffusion decay time of the inspected medium, on the order of hundreds of μs. An external neutron detector which is designed to detect only epithermal neutrons, will measure only a single decaying exponential when there is no SNM present, and two exponentials when SNM is present. This paper shows that in many cases, a gain in detection sensitivity can be realized by introducing a thermal neutron poison (such as boron) into the detector. This poison will reduce the efficiency of the detector, but decrease its decay time. A decreased decay time will cause the separation between the detector and fission signal exponentials to occur at an earlier time. There is a balance between efficiency and time constant for a detector. The boron concentration to achieve the maximum sensitivity, and its magnitude, will be different for different detector designs.

Remote thermal neutron die-away measurements to improve Differential Die-Away Analysis

Differential Die-away Analysis (DDAA) is a very effective detection technique of special nuclear material (SNM). It is based on the detection of fast fission neutrons that decay with a time constant characteristic of the thermal neutron that created them. The presence of fast neutrons with this slow die-away time is a positive, sensitive indication of the presence of SNM in the inspected cargo. The thermal decay time of an inspected medium is not known before a DDAA measurement is taken. The choice of when, from the end of a neutron pulse, data are to be acquired must be taken conservatively. If the thermal decay time of an inspected medium can be determined, a priori, then a better estimate of the expected time of separation between the intrinsic detector signal and the fission signal can be achieved. With a better estimate of this time, the choice of times after the end of the pulse to perform signal integration can be optimized. A more optimal choice for this time will result in higher detection sensitivity. This paper describes a method of remotely assessing the thermal die-away properties of an interrogated cargo, and grossly characterizes the class of material being inspected in terms of neutron attenuation and moderation properties.

Detection of 235U in hydrogenous cargo with Differential Die-Away Analysis and optimized neutron detectors

Differential Die-Away Analysis (DDAA) is a sensitive technique for detecting the presence of fissile materials such as 235U and 239Pu. In DDAA, a neutron generator produces repetitive pulses of neutrons that are directed into a cargo being inspected. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The thermalization process is very rapid and the population of the source and epithermal neutrons decays away within microseconds. The population of thermal neutrons, however, decays much slower with the diffusion decay time of the inspected medium (thermal neutron “die away” time), on the order of hundreds of microseconds. If Special Nuclear Material (SNM) is present, the thermalized neutrons from the source will cause fissions that produce a new source of neutrons. These fast fission neutrons decay with a time very similar to that of the thermal neutron die-away of the surrounding cargo. The sensitivity of DDAA for a given source of neutrons critically depends on optimizing the fast/epithermal neutron detection system. Optimization involves both time response and detection efficiency. Optimized detectors were able to detect fissile material throughout pallets of cargo.